Why does this NLA strip have no action, and how do I fix it?
Here is the part in the manual that concerns you
Also here is a link to a tutorial on the NLA

Deleting &amp; Converting

If you decide that you no longer want a stashed action anymore, simply delete the corresponding NLA strip, then save and reload the file.

Also, note that since these are NLA strips, you can reuse these as normal NLA strips simply by un-muting (and renaming) the NLA track they live in. You may also want to move it above all the other stashed-action tracks.   

Remove Empty Animation Data

This operator removes AnimData data-blocks (restricted to only those which are visible in the animation editor where it is run from) which are “empty” (i.e. that is, have no active action, drivers, and NLA tracks or strips).

It is sometimes possible to end up with a lot of data-blocks which have old and unused Animation Data containers still attached. This most commonly happens when doing motion graphics work (i.e. when some linked-in objects may have previously been used to develop a set of reusable assets), and is particularly distracting in the NLA Editor.